The present invention relates to a memory display system. With this system a latent image is formed first of all, which is developed by electric stimulation, in the form of a light ray at a wavelength generally greater than that used for recording. More precisely, this device relates to X ray image formation, with sequential type electric access.
The image forming process which we propose is intended to replace the photographic film, still widely used today in numerous fields, and in particular for X ray image formation. In the case where the system proves fairly rapid so as to be able to make several images per second, it may also replace the radiological image intensifier tube, in the same field as X ray image formation.
It should be recalled that the photographic film, although it has numerous advantages (sensitivity, resolution) which justify its use as image detector nevertheless has several drawbacks related on the one hand to the physical properties, namely reduced dynamics, and on the other to the nature of the detector, namely handling of the plate required for its development.
In addition, once the image is obtained on the film medium, digitization of the image with a view to subsequent processing involves an additional step requiring the use of a microdensitor meter.
Furthermore, the film is both image detector and final support medium for the image. Now, an image forming system, the performances required for the detector (dynamic/sensitivity) and those required for the support medium (strong contrast and low noise) are, to a certain extent, antagonistic. Thus, the film appears as a compromise, which necessarily sacrifices the performances of both functions.
Other image forming methods have seen the light of day, in an attempt to overcome these difficulties. There may be mentioned, for X ray image formation, on the one hand, the electrostatic or zero radiographic image formation techniques and on the other a digitized image formation technique in which a luminescent material photostimulable by laser is used as storage material for the temporary storage of the X ray image, which is described in the document "Radiology" 148, 833, Sept. 1983.
Xeroradiography gives images whose contours are strongly contrasted. This is due to the non linearity of development. On the other hand, this technique is about 20 times less sensitive to high energies than the best image forming techniques using a film-intensifying screen combination; the exposure times are consequently higher. At the present time, this technique is restricted to observation of the breast and hands and feet.
The method described in the above mentioned document has all the advantages of film image formation without having the drawbacks thereof: the sensitive memory plate and the final support medium for the image are separated. Applied to X ray image formation, the system has the resolution of the photographic film, with dynamics and sensitivity greater than those of the film. The system further allows the dose of radiation applied to the patient to be reduced with respect to systems using film. In addition, the images may be processed by computer.
As for the film, the two following drawbacks must attract attention. On the one hand, it is necessary to handle the cassette so as to develop the stored image. Acquisition of the image cannot be made in situ. On the other hand, the procedure for reading the sensitive plate by laser is time consuming: in fact, the time for acquiring an image is of the order of a minute. This does not allow the rapid taking of pictures which are interesting for following up transitory phenomena.
This is why the invention relates to a system allowing these drawbacks to be overcome and more particularly a memory image forming system which can be read in situ, without any handling.